A. Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) refers generally to a form of clinical imaging based upon the principles of nuclear magnetic resonance (NMR). Any nucleus which possesses a magnetic moment will attempt to align itself with the direction of a magnetic field, the quantum alignment being dependent, among other things, upon the strength of the magnetic field and the magnetic moment. In MRI, a uniform magnetic field B.sub.0 is applied to an object to be imaged; hence creating a net alignment of the object's nuclei possessing magnetic moments. If the static field B.sub.0 is designated as aligned with the z axis of a Cartesian coordinate system, the origin of which is approximately centered within the imaged object, the nuclei which posses magnetic moments precess about the z-axis at their Larmor frequencies according to their gyromagnetic ratio and the strength of the magnetic field.
Water, because of its relative abundance in biological tissues and its relatively strong net magnetic moment M.sub.z created when placed within a strong magnetic field, is of principle concern in MR imaging. Subjecting human tissues to a uniform magnetic field will create such a net magnetic moment from the typically random order of nuclear precession about the z-axis. In a MR imaging sequence, a radio frequency (RF) excitation signal, centered at the Larmor frequency, irradiates the tissue with a vector polarization which is orthogonal to the polarization of B.sub.0. Continuing our Cartesian coordinate example, the static field is labeled B.sub.z while the perpendicular excitation field B.sub.1 is labeled B.sub.xy. B.sub.xy is of sufficient amplitude and duration in time, or of sufficient power to nutate (or tip) the net magnetic moment into the transverse (x-y) plane giving rise to M.sub.xy. This transverse magnetic moment begins to collapse and re-align with the static magnetic field immediately after termination of the excitation field B.sub.1. Energy gained during the excitation cycle is lost by the nuclei as they re-align themselves with B.sub.0 during the collapse of the rotating transverse magnetic moment M.sub.xy.
The energy is propagated as an electromagnetic wave which induces a sinusoidal signal voltage across discontinuities in closed-loop receiving coils. This represents the NMR signal which is sensed by the RF coil and recorded by the MRI system. A slice image is derived from the reconstruction of these spatially-encoded signals using well known digital image processing techniques.